In large industrial or utility motors and generators, the stator windings, also known as armature windings, are inspected from time to time to confirm the integrity of the insulation. Each stator winding includes a conductive bar(s) wrapped in layers of insulation. The insulation confines the current in the bars to the bars, prevents arcing of current between windings, and shields the bars against stray objects that could electrically short the bars and to protect people and equipment. In view of the high current levels that flow through industrial generators and motors, the insulation on stator bars must provide an effective and complete barrier surrounding the bars. If the insulating properties of the insulation degrades because it becomes damp or for other reasons, then voltage arcs may jump from the stator bars through degraded regions of the insulation to cause electrical shorts that can harm people and damage equipment.
The insulation on stator bar windings are inspected from time to time to determine whether the insulation has degraded and, if so, to what extent. Stator bars on older water-cooled generators are especially susceptible to water leaks and must undergo regular periodic inspection and testing. One test which has proven very reliable in identifying stator bars with deteriorated groundwall insulation is a capacitance "mapping" of the stator bars. Since the dielectric constant of an insulator provides a measure of its insulating properties, the insulation of an stator bar winding can be inspected by determining the dielectric constant of the insulation. The dielectric constant of the insulator can be calculated by measuring the capacitance of the insulation on the stator bars. The dielectric constant indicates such conditions as the amount of dampness in the insulator. A damp insulator may indicate a leak in the water passages in a water cooled stator. A damp insulator may be water damaged and not functioning as an effective insulator.
All insulating materials have a dielectric constant, which is a measure of the amount of energy the insulating material stores when a voltage is applied across the material. The approximate dielectric constant for air is 1.0 and 80 for water. However, for Micapal.TM., a common insulation material for stator windings, it is approximately 4 (for undamaged Micapal.TM.). Where there is a mixture of air, water and insulation, the measured capacitance will be a different composite number. Because of the large difference in the dielectric constants for Micapal.TM. (and other winding insulators) and water, the dielectric constant changes relatively dramatically when an insulator for a winding becomes damp. Accordingly, measuring the dielectric constant of an insulator provides an effective means for detecting water logged insulation on stator windings.
The dielectric constant for a stator insulator can be calculated using capacitance values measured across the insulator. Capacitance and the dielectric constant are related as described in the following equation:
C=kDA/t
Where: D=dielectric constant of the insulation
A=area of the probe electrode PA1 t=thickness of the insulation
Because the area (A) of a probe electrode and thickness (t) of the insulator are know quantities and the capacitance (C) of the insulator is a measured quantity, the dielectric constant (D) can be relatively easily calculated with the above equation. A meter is used to measure the capacitance across the insulation between the electrode and the stator bar conductor. Each stator bar in the winding is measured at both ends of the core and statistical analysis is used to identify those bars with higher than normal expected capacitance. A relatively higher capacitance is a good indication of moisture present in the insulation.
To obtain accurate capacitance measurements of the insulation on stator bars, the capacitance measurement probe must be precisely inserted into the proper position between adjacent stator bars and the electrode placed firmly against the insulated surface of the particular stator bar measured. In this manner, a "mapping" of insulation capacitance for a particular motor or generator could be compiled and used to identify stator bars with damaged or faulty insulation. Consequently, a special capacitance probe having an inflatable bladder was developed for improving the accuracy and ease with which such measurements are made. The special probe developed is the subject of commonly assigned U.S. Pat. No. 5,546,008 issued Aug. 13, 1996, to Sminchak et al., entitled "INFLATABLE CAPACITANCE MEASURING DEVICE", which is incorporated by reference herein. Unfortunately, capacitance mapping using such a probe required disassembly of the generator and removal of the field producing component (i.e., the rotor) so that the stator bars were accessible to a service technician for inserting and properly positioning the probe at the desired positions between adjacent stator bars. Since removal of the field from a large motor or power generator is an expensive and time consuming process, capacitance mapping was usually a last resort maintenance or inspection procedure.